Heretofore, a polyester has been widely used for various applications to e.g. containers such as bottles, films, sheets and fibers, since it is excellent in chemical and physical properties. Such a polyester is usually produced via an esterification reaction or an ester exchange reaction and a melt polycondensation reaction and as the case requires, particularly for containers, further by a solid phase polycondensation reaction, and for the polycondensation reaction, a compound such as antimony, germanium or titanium is used as a catalyst.
However, a polyester produced by employing an antimony compound as a catalyst has characteristic dull color and has drawbacks in the safety and sanitation and in the environment due to toxicity pointed out regarding the antimony compound. Accordingly, it has been strongly required to reduce the amount of the antimony compound used or to develop a polycondensation catalyst which takes the place of the antimony compound. Further, although a polyester produced by employing a germanium compound as a catalyst is preferred in view of e.g. transparency and safety and sanitation, the germanium compound itself is very expensive and economical disadvantage is inevitable. Therefore, it has also been strongly desired to reduce the amount of the germanium compound used or to develop a polycondensation catalyst which takes the place of it. Under these circumstances, a titanium compound has attracted attention which is available at a low cost and is free from problems in view of e.g. safety and sanitation, and has been used practically instead of the antimony compound or the germanium compound. However, a polyester produced by employing a titanium compound as a catalyst has such drawbacks that it is characteristically yellowish, and is poor in the thermal stability.
To solve such drawbacks caused by a titanium compound, many proposals have been made to use a titanium compound, an alkali metal or alkaline earth metal compound and a phosphorus compound in a specific ratio, thereby to improve e.g. the color tone of a polyester to be obtained (for example, JP-A-2002-179781). However, such a catalyst system is poor in catalytic activity, and thus the polycondensation requires a long time or a relatively large amount of a titanium compound is required, whereby the color tone deteriorates and as a result, the color tone can be by no means improved.
Further, many proposals have been made regarding use of a composite compound comprising a titanium compound and another metal compound or the like. For example, it is proposed that by use of a solid titanium-containing compound obtained by hydrolyzing a mixture comprising a titanium alkoxide compound or a titanium halide with a compound such as magnesium, the catalytic activity of the titanium compound will improve and in addition, the transparency and the color tone of a polyester to be obtained will improve (JP-A-2001-55434, JP-A-2001-64377).
Further, it has been known that a high catalytic activity will be obtained by use of a co-precipitate obtained by simultaneous hydrolysis of an alkylate, alcoholate or carboxylate of titanium and an alkylate, alcoholate or carboxylate of an alkali metal, an alkaline earth metal or the like (JP-A-2002-503274). However, according to studies by the present inventors, it was found that the composite compound comprising a titanium compound and another metal compound or the like proposed in the publications is in a solid state, and is insoluble in an organic solvent of e.g. a diol component as a raw material for the polyester, and when it is used in the form of a slurry having it dispersed in such an organic solvent (the slurry had a turbidity exceeding 20% in a light path length of 10 mm), the polycondensation reaction rate tends to be insufficient and further, there is a problem in e.g. handling efficiency. Further, it was found that various physical properties such as color tone and transparency of a polyester to be obtained should be further improved.